Immunologic memory is the hallmark of the adaptive immune system, and is critical for vaccine efficacy as well as long-term natural pathogen resistance. Memory is an emergent property of the immune system as a whole, stemming from the interplay of increased precursor frequencies among T and B cells along with differentiation of these cells such that they differ from their naive counterparts in sensitivity to Ag signals, costimulatory signals as well as in the quality of the responses to such stimuli. Though it is relatively easy to detect the presence of functional memory, it has been much more difficult to understand the basis for this at the cellular level. A major obstacle is that memory B cells are relatively rare and difficult to identify. We have recently developed a system in which we can demonstrate the properties of memory B cells. In this IgH Tg mouse system, the precursor frequency of naive Ag-specific cells is approximately 3%, leading to extremely robust primary responses and the consequent formation of very large numbers of long-lived memory B cells. Sixteen weeks after a second immunization, these cells comprise 25 million memory cells per spleen, compared with less than 100,000 in a non-Tg animal, and which is ten-fold higher than in naive animals. Thus, the population of Ag-specific cells is mainly (90-95%) comprised of memory cells with a small admixture of naive cells. This allows us to make valid comparisons with 100% naive populations without further "purifying" the memory cells based on any presuppositions or "memory markers" that can be fraught with difficulty. The ability to isolate this quantity of Ag-specific memory cells, along with their naive counterparts, allows studies of memory cells which would otherwise not be possible (and have not been done to date, to our knowledge). Specifically, we propose to use the system to ask the following questions: Aim 1. What makes a resting B memory cell a different from a naive cell? Aim 2. How do memory B cells react differently from naive cells to relevant stimuli? Aim 3. What are the unique survival factor requirements, migration properties, and responses to immunization of memory cells in vivo? Progress in answering these questions advances important basic knowledge in immunobiology and also has a great degree of relevance to vaccine design and clinical issues like the waning of immunity with age.